Nest 3rd Gen Problem - Hot Water comes on with Heating

[Peter007]

Morning all,

I was hoping somebody could help me with a Nest 3rd gen installation please? My problem is that the Hot Water comes on with the Heating, even when it's set to off from the app.

My set up was:
Boiler: Vaillant EcoTec Plus 630
Tank: Megaflow 210l
Programmer: Honeywell ST9400c
Wirelesss Thermostat and Receiver: Honeywell Y6630D with Thermostat in the hallway and receiver next to the Megaflow tank in the airing cupboard.

I followed a previous post to wire up the Nest Heatlink which I thought was similar to my set up:
//www.diynot.com/diy/threads/nest-heatlink-wiring-to-replace-honeywell.453249/

Things I've done: Wire up the Nest Heatlink to replace the St9400c programmer, Bridge the Live to the Common for CH and HW.

My previous St9400c wiring and Nest Heatlink wiring pics are attached.

Would very much appreciate some advice from the experts

 

[Peter007]

Pics attached

 

[stem]

Hi Peter007 and welcome to the forum.

The wiring in the photos that you show looks correct.

From the ST9400 the hot water 'on' grey wire went to terminal 3, which corresponds with the Nest terminal 6
From the ST9400 the heating 'on' black wire went to terminal 4, which corresponds with the Nest terminal 3
The links between L, 2 and 5 are also correct

So, first of all did the system work correctly before you started to install the Nest?

What you show is only part of the modified wiring, can you show us all the other changes you have made. For example how did you remove the existing Honeywell room thermostat and modify the wiring to take into account its removal?

 

[Peter007]

Hi Stem,

Thanks for the reply - glad to hear the Nest wiring looks correct! I haven't removed the Honeywell room thermostat or receiver. Its still in place with the thermostat turned to max. I didn't want to remove it until I was sure the Nest is wired up and working correctly.

Before fitting the Nest everything was fine, so as I've only replaced the programmer with the Nest. What's strange is that when the Nest is set to heat, the hotwater shows as off but it is still being heated as it comes out hot from the taps.

I've attached the Megaflow wiring pic and Honeywell receiver pic if its helps shed some light - but these have remained untouched.

 

[Peter007]

Pics of the Megaflow wiring and Honeywell receiver attached

 

[stem]

the hotwater shows as off but it is still being heated as it comes out hot from the taps.

That's normal. Once the megaflow is heated, you will have 210 liters of stored hot water sitting in your hot water cylinder, which depending on how you use it, can will last for days without being reheated.

I haven't removed the Honeywell room thermostat or receiver. Its still in place with the thermostat turned to max.

OK that's fine. No other changes to the wiring then.

 

[Peter007]

That's correct, no other changes. Maybe everything is working as normal then and I'm just being paranoid due having touched any wiring

 

[stem]

The wiring you have changed is fine and as it should be.

Once you are happy with it, I would recommend that you consider removing the existing thermostat, otherwise, you will still need to change the batteries in it from time to time.

 

[Peter007]

Thanks Stem for having a look at my issue and your advice - much appreciated.